1. Field of the Invention
The present invention relates to a method for manufacturing a moisture permeable electrode for use in a moisture sensing element.
2. Description of the Prior Art
A moisture sensing element has been developed, which includes a moisture sensitive thick-film or a moisture sensitive thin-film made of, for example, a polymer film or a metal oxide film. A pair of electrodes are formed on the opposing major surfaces of the moisture sensitive film so as to detect the impedance representative of the humidity. In such a construction, at least one of the pair of electrodes must be a moisture permeable electrode to ensure the moisture detecting operation.
A gold film is widely used as the electrode in a moisture sensing element. The gold film has a low resistance, and shows a high water resisting property and a high chemical resistance. When the moisture sensing element has a ceramic substrate such as an alumina substrate, the irregularities of the ceramic substrate form irregularities on the surface of the moisture sensitive film without regard to the material of the moisture sensitive film. The thus formed irregular surface provides clearances between the moisture sensitive film and an electrode film formed on the moisture sensitive film, the clearances providing the moisture permeability.
When the moisture sensitive film is made of ceramics such as a metal oxide, the moisture sensitive film itself has an irregular surface. Therefore, a moisture permeable electrode can be formed on the moisture sensitive film. That is, if the moisture sensitive film has an irregular surface, a moisture permeable electrode is formed on the moisture sensitive film. In this case, the material of the electrode film, the thickness of the electrode film, and the deposition condition of the electrode film are not necessarily controlled strictly.
However, if a moisture sensing element has a flat substrate such as a glass substrate, and a flat moisture sensitive film such as a polymer film, preferred moisture permeability is not ensured when the electrode is made of a gold film or a gold alloy film. If the thickness of the electrode film is selected considerably thin, clearances are formed in the electrode film, which provide moisture permeability. However, the specific resistance greatly increases even when the thickness slightly reduces in case the gold electrode film has a thickness less than 100 .ANG.. That is, it is difficult to accurately control the specific resistance of a thin gold film, and the reproducibility of the electrode film is very low.